This invention relates to digital computer network technology. More specifically, it relates to methods and apparatus for synchronizing components within the Head End of an access network.
Broadband access technologies such as cable, fiber optic, and wireless have made rapid progress in recent years. Recently there has been a convergence of voice and data networks which is due in part to US deregulation of the telecommunications industry. In order to stay competitive, companies offering broadband access technologies need to support voice, video, and other high-bandwidth applications over their local access networks. For networks that use a shared access medium to communicate between subscribers and the service provider (e.g., cable networks, wireless networks, etc.), providing reliable high-quality voice/video communication over such networks is not an easy task.
A cable modem network or “cable plant” employs cable modems, which are an improvement of conventional PC data modems and provide high speed connectivity. Cable modems are therefore instrumental in transforming the cable system into a full service provider of video, voice and data telecommunications services. Digital data on upstream and downstream channels of the cable network is carried over radio frequency (“RF”) carrier signals. Cable modems convert digital data to a modulated RF signal for upstream transmission and convert downstream RF signal to digital form. The conversion is done at a subscriber's facility. At a Cable Modem Termination System (“CMTS”), located at a Head End of the cable network, the conversions are reversed. The CMTS converts downstream digital data to a modulated RF signal, which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the digital data is fed to the cable modem (from an associated PC for example), which converts it to a modulated RF signal. Once the CMTS receives the upstream RF signal, it demodulates it and transmits the digital data to an external source.
FIG. 1 is a block diagram of a typical two-way hybrid fiber-coaxial (HFC) cable network system. It shows a Head End 102 (essentially a distribution hub) which can typically service about 40,000 homes. Head End 102 contains a CMTS 104 that is needed when transmitting and receiving data using cable modems. Primary functions of the CMTS include (1) receiving baseband data inputs from external sources 100 and converting the data for transmission over the cable plant (e.g., converting Ethernet or ATM baseband data to data suitable for transmission over the cable system); (2) providing appropriate Media Access Control (MAC) level packet headers for data received by the cable system, and (3) modulating and demodulating the data to and from the cable system.
Head End 102 connects through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108. Each Head End can support normally up to 80 fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the Head End and each distribution node. In addition, because cable modems were not used, the Head End of pre-HFC cable systems did not contain a CMTS. Returning to FIG. 1, each of the fiber nodes 108 is connected by a coaxial cable 70 to two-way amplifiers or duplex filters 72, which permit certain frequencies to go in one direction and other frequencies to go in the opposite direction (different frequency ranges are used for upstream and downstream paths). Each fiber node 108 can normally service up to 500 subscribers. Fiber node 108, coaxial cable 70, two-way amplifiers 72, plus distribution amplifiers 74 along with trunk line 76, and subscriber taps, i.e. branch lines 78, make up the coaxial distribution system of an HFC system. Subscriber tap 78 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a subscriber computer 122.
In order for data to be able to be transmitted effectively over a wide area network such as HFC or other broadband computer networks, a common standard for data transmission is typically adopted by network providers. A commonly used and well known standard for transmission of data or other information over HFC networks is DOCSIS. The DOCSIS standard has been publicly presented by Cable Television Laboratories, Inc. (Louisville, Colo.) in document control number SP-RFIv1.1-I02-990731, Jul. 31, 1999. That document is incorporated herein by reference for all purposes.
Data Communication in Cable Networks
In conventional DOCSIS systems, the CMTS may include a plurality of physically distinct line cards having appropriate hardware for communicating with cable modems in the network. Each line card is typically assigned to a separate DOCSIS domain, which is a collection of downstream and upstream channels for which a single MAC Allocation and Management protocol operates. Typically, each DOCSIS domain includes a single downstream channel and one or more upstream channels. The downstream channel is used by the CMTS to broadcast data all cable modems (CMs) with that particular domain. Only the CMTS may transmit data on the downstream. In order to allow the cable modems of a particular DOCSIS domain to transmit data to the CMTS, the cable modems share one or more upstream channels within that domain. Access to the upstream channel is controlled using a time division multiplexing (TDM) approach. Such an implementation requires that the CMTS and all cable modems sharing an upstream channel within a particular domain have a common concept of time so that when the CMTS tells a particular cable modem to transmit data at time T, the cable modem understands what to do. “Time” in this context is tracked using a counter, commonly referred to as a timestamp counter, which, according to conventional implementations is a 32-bit counter that increments by one every clock pulse.
In conventional CMTS configurations, each line card in the system includes a separate MAC controller which is responsible for implementing a DOCSIS MAC protocol between the CMTS and the cable modems serviced by that particular line card. Each MAC controller has its own unique timestamp counter which generates its own local time reference. Thus, each line card in the system operates according to its own local time reference, and is not synchronized with other line cards in the system. Further, each line card in the system periodically distributes a timestamp value of its local time reference to the respective group of cable modems serviced by that line card. For this reason, a first group of cable modems serviced by a first line card will not be in synchronization with a second group of cable modems serviced by a second line card at the CMTS. While such a configuration provides for simplicity in terms of implementation, it may not be the most advantageous configuration for handling new and emerging broadband network applications such as video-on-demand, telephony, etc. Accordingly, there exists a continual need to improve access network configurations in order to accommodate new and emerging network applications and technologies.